I don’t recommend this type of setup, it’s very complicated and if something doesn’t work just right you won’t have mail. Get a business class line and static IP if you want to host at home.
While I concur with Chris completely, I’ll still answer the question. You can check the ports simply by reviewing what MiaB allows through the firewall by running ufw status from the command line.
Wouldn’t it be simpler and more reliable to just combine your 3 instances into 1?